[ISSUE #10939] Validate missing Remoting ext fields before forwarding

[Aias00]

What does this PR do?

Returns the existing VERSION_NOT_SUPPORTED protocol response when a Proxy Remoting request has no extFields map, instead of dereferencing it and failing with a NullPointerException.

Test

• mvn -pl proxy -Dtest=AbstractRemotingActivityTest test
• Adds coverage for normal forwarding and SEND_MESSAGE_V2 requests with extFields == null.

Closes #10939.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.49%. Comparing base (293f588) to head (c99a669).

Files with missing lines	Patch %	Lines
...xy/remoting/activity/AbstractRemotingActivity.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #10940      +/-   ##
=============================================
- Coverage      48.62%   48.49%   -0.14%     
+ Complexity     13692    13649      -43     
=============================================
  Files           1381     1381              
  Lines         101464   101464              
  Branches       13187    13187              
=============================================
- Hits           49337    49205     -132     
- Misses         46142    46243     +101     
- Partials        5985     6016      +31     

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[RockteMQ-AI]

Summary

Good null safety fix for validating Remoting ext fields before accessing them.

The change adds null checks for request.getExtFields() before calling .get() on it, preventing NullPointerException when a malformed request without ext fields is received. This is a defensive fix that improves robustness against invalid client requests.

The fix is applied to both code paths (SEND_MESSAGE_V2 and other request types), and the test coverage validates both scenarios with null ext fields.

LGTM.

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